The essence of adaptive immunity is the ability of an organism to react to the presence of foreign substances and produce components (antibodies and cells) capable of specifically interacting with and protecting the host from their invasion. An “antigen” or “immunogen” is a substance that is able to elicit this type of immune response and also is capable of interacting with the sensitized cells and antibodies that are manufactured against it.
Antigens or immunogens are usually macromolecules that contain distinct antigenic sites or “epitopes” that are recognized and interact with the various components of the immune system. They can exist as individual molecules composed of synthetic organic chemicals, proteins, lipoproteins, glycoproteins, RNA, DNA, or polysaccharides, or they may be parts of cellular structures (bacteria or fungi) or viruses (Harlow and Lane 1988a, b, c; Male et al., 1987).
Small molecules like short peptides, although normally able to interact with the products of an immune response, often cannot cause a response on their own. These peptide immunogens or “haptens” as they are also called, are actually incomplete antigens, and, although not able by themselves to cause immunogenicity or to elicit antibody production, can be made immunogenic by coupling them to a suitable carrier. Carriers typically are protein antigens of higher molecular weight that are able to cause an immunological response when administered in vivo.
In an immune response, antibodies are produced and secreted by the B-lymphocytes in conjunction with the T-helper (TH) cells. In the majority of hapten-carrier systems, the B cells produce antibodies that are specific for both the hapten and the carrier. In these cases, the T lymphocytes will have specific binding domains on the carrier, but will not recognize the hapten alone. In a kind of synergism, the B and T cells cooperate to induce a hapten-specific antibody response. After such an immune response has taken place, if the host is subsequently challenged with only the hapten, usually it will respond by producing hapten-specific antibodies from memory cells formed after the initial immunization.
Synthetic haptens mimicking some critical epitopic structures on larger macromolecules are often conjugated to carriers to create an immune response to the larger “parent” molecule. For instance, short peptide segments can be synthesized from the known sequence of a protein and coupled to a carrier to induce immunogenicity toward the native protein. This type of synthetic approach to the immunogen production has become the basis of much of the current research into the creation of vaccines. However, in many instances, merely creating a B-cell response by using synthetic peptide-carrier conjugates, however well designed, will not always guarantee complete protective immunity toward an intact antigen. The immune response generated by a short peptide epitope from a larger viral particle or bacterial cell may only be sufficient to generate memory at the B cell level. In these cases it is generally now accepted that a cytotoxic T-cell response is a more important indicator of protective immunity. Designing peptide immunogens with the proper epitopic binding sites for both B-cell and T-cell recognition is one of the most challenging research areas in immunology today.
The approach to increasing immunogenicity of small or poorly immunogenic molecules by conjugating these molecules to large “carrier” molecules has been utilized successfully for decades (see, e.g., Goebel et al. (1939) J. Exp. Med. 69: 53). For example, many immunogenic compositions have been described in which purified capsular polymers have been conjugated to carrier proteins to create more effective immunogenic compositions by exploiting this “carrier effect.” Schneerson et al. (1984) Infect. Immun. 45: 582-591). Conjugation has also been shown to bypass the poor antibody response usually observed in infants when immunized with a free polysaccharide (Anderson et al. (1985) J. Pediatr. 107: 346; Insel et al. (1986) J. Exp. Med. 158: 294).
Hapten-carrier conjugates have been successfully generated using various cross-linking/coupling reagents such as homobifunctional, heterobifunctional, or zero-length cross linkers. Many such methods are currently available for coupling of saccharides, proteins, and peptides to peptide carriers. Most methods create amine, amide, urethane, isothiourea, or disulfide bonds, or in some cases thioethers. A disadvantage to the use of coupling reagents, which introduce reactive sites in to the side chains of reactive amino acid molecules on carrier and/or hapten molecules, is that the reactive sites if not neutralized are free to react with any unwanted molecule either in vitro (thus adversely affecting the functionality or stability of the conjugate(s)) or in vivo (thus posing a potential risk of adverse events in persons or animals immunized with the preparations). Such excess reactive sites can be reacted or “capped”, so as to inactivate these sites, utilizing various known chemical reactions, but these reactions may be otherwise disruptive to the functionality of the conjugates. This may be particularly problematic when attempting to create a conjugate by introducing the reactive sites into the carrier molecule, as its larger size and more complex structure (relative to the hapten) may render it more vulnerable to the disruptive effects of chemical treatment. In fact, no examples are known of methods whereby a conjugate is made by first activating the carrier, then reacting with the hapten in a conjugation reaction, and finally “capping” the remaining reactive sites, while preserving the ability of the resulting conjugate to function as an immunogenic composition having the desired properties of the “carrier effect”.